A subsea well of the type concerned herein will have a wellhead supported on the subsea floor. One or more strings of casing will be lowered into the wellhead from the surface, each supported on a casing hanger. The casing hanger is a tubular member that is secured to the threaded upper end of the string of casing. The casing hanger lands on a landing shoulder in the wellhead, or on a previously installed casing hanger having larger diameter casing. Cement is pumped down the string of casing to flow back up the annulus around the string of casing. Afterward, a packoff is positioned between the wellhead bore and an upper portion of the casing hanger. This seals the casing hanger annulus.
One type of packoff utilizes a metal seal so as to avoid deterioration with time that may occur with elastomeric seals. Metal seals require a much higher force to set than elastomeric seals. Prior art running tools have employed various means to apply the downward force needed to set a packoff. Some prior art tools use rotation of the drill string to apply setting torque. It is difficult to achieve sufficient torque to generate the necessary forces for a metal packoff, because the running tool may be located more than a thousand feet below the water surface in deep water.
Other running tools and techniques shown in the patented art apply pressure to the annulus below the blowout preventer and the running tool. If the blowout preventer is at the surface, the amount of annulus pressure is limited, however, to the pressure rating of the riser through which the drill string extends. This pressure rating is normally not enough to set a metal packoff.
Higher pressure can be achieved by pumping through the drill string. However, this requires a running tool with some type of ports that are opened and closed from the surface. This is necessary because cement must first be pumped down the drill string. The ports may be open and closed by dropping a ball or dart. A considerable amount of time, however, is required for the ball to reach the seat. Rig time is quite expensive. Another method employs raising and lowering the drill pipe and rotating in various manners to engage and disengage J-slots to open and close ports. This has a disadvantage of the pins for the J-slots wearing and not engaging properly.
As previously indicated, often times a portion of drill pipe must be sealed in order to pressurize the volume of pipe above the seal. In many instances an object such as a ball, a dart, or a plug, is dropped down the drill pipe to create a seal which isolates the area above the object, allowing it to be pressurized. In order to create a seal, there must be a surface within the drill pipe for the object to land on and seal against. The seal is then deactivated by over-pressurizing, which can burst a rupture disc, break shear pins, or extrude metal. Alternatively, the object can be retrieved on a wire line. In other instances, a plug may be preinstalled prior to running the tool. However, in this instance, once the drill pipe has been pressurized, the plug must be deactivated as previously discussed. The dropping and retrieval of the sealing object is time consuming and often proves to be unreliable and inconsistent.
A need exists for a technique that addresses the effective and efficient activation and deactivation of a seal for isolating and pressurizing a section of drill pipe. The following technique may solve one or more of these problems.